In search of geologic formations likely to trap oil or gas, the offshore seismic exploration industry surveys the outer layers of the earth's crust beneath the ocean by towing an array of hydrophones behind a boat, periodically firing a source of acoustic energy, recording the responses of the hydrophones to reflections of the acoustic energy from geologic formations, and processing the seismic hydrophone data. The hydrophone array is linearly arranged in a streamer whose depth is controlled. The streamer, which may be a few kilometers long, may also include a head buoy tethered to the head end of the streamer and a tail buoy to the tail end as surface references.
Historically, only one streamer containing the hydrophone array was deployed from the exploration boat during a survey. The accuracy of the survey depended on, among other things, the accuracy of the estimate of the shape of the hydrophone streamer and the accuracy of the positioning of a known point on the streamer.
One way the shape can be estimated is by mechanically modeling the streamer and computing its dynamic performance under various towing speeds and ambient conditions. The accuracy of the estimation is, of course, only as good as the model. Placing magnetic compasses and depth sensors along the streamer represented an improvement in streamer shape estimation. Data representing the depth and magnetic heading of sections of the streamer are sent from the distributed compasses and depth sensors to a controller on board the tow boat for immediate computation of streamer shape and for storage of the raw data for later detailed processing. Accurate shape estimation is achieved in this way.
As important as estimating the streamer's shape is tying its position to a geodetic reference. Typically, radiopositioning receivers aboard the boat are used to tie a spot on the boat to a geodetic reference. Accurate optical positioning systems, such as a laser, are then used to tie the front buoy to the geodetic reference. It is also common to have a radiopositioning receiver aboard the tail buoy to fix its position. The positions of the distributed compasses and depth sensors with respect to the buoys is then estimated based on a model of the streamer and the buoy tethers. Inaccuracies in the model result in absolute errors in transferring the geodetic reference from the buoys to the streamer. Furthermore, the performance of optical positioning systems degrades with inclement weather.
An important advance in the exploration for oil and gas is the development of the three-dimensional seismic survey, often using more than one hydrophone streamer. With multiple streamers towed behind one or more boats, more seismic hydrophone data can be gathered in much less time than with a single streamer, resulting in a significant reduction in exploration costs. With multiple streamers, accurate estimations of the positions of the hydrophone streamers with respect to each other and to the acoustic source are essential. Fortunately, multiple streamers towed more or less in parallel provide a geometry favorable for determining the positions of the streamers with respect to each other, to the boat, to the acoustic source, or gun, and to the buoys by means of acoustic ranging. With individual hydroacoustic transceivers positioned along the streamers, on the acoustic source, on the boat or boats, and on the buoys, acoustic transit times of pulses transmitted by the transceivers and received by neighboring transceivers can be telemetered to the controller on the boat where a position solution can be performed and the raw data stored for further processing. Using the speed of sound through water, the controller converts the transit times into spatial separations between pairs of transceivers in developing the position solution. With information from a radiopositioning system and from depth sensors and compasses positioned along the array, the position solution is complete.
In a typical three-dimensional survey run using more than one streamer, the towing boat or boats follow a more or less constant heading at a more or less constant speed through the survey field. Waves, wind, current, and inevitable variations in boat speed and heading continuously affect the shapes of the streamers. Periodically, for example, every ten seconds, the acoustic source, or gun, is fired. An impulse of compressed air is forced into the water creating a bubble. The collapse of the bubble causes an acoustic pulse that radiates through the water and into the earth. Reflections of the pulse off geologic structures are picked up by the hydrophones and data representing these reflections are sent to the controller on the boat. Each firing of the gun and the associated interval during which the acoustic echoes are detected is known as a shot point. It is important that data sufficient to perform a complete position solution for each shot point be available. For a group of long streamers with acoustic transceivers distributed along each, many acoustic ranges must be measured. In theory, it would be best if all of the ranges to be measured could be determined simultaneously before the streamer has a chance to change its shape and position. Unfortunately, that is not possible in practice. The idea, then, is to measure all the acoustic ranges in as little time as possible, which requires a high throughput for each transceiver.
The separation between a pair of transceivers is generally measured by either one-way or two-way ranging. In one-way ranging, the first transceiver transmits a hydroacoustic pulse at time t.sub.s. The pulse propagates through the water where it is received by the other transceiver at time t.sub.r. The time difference t.sub.r -t.sub.s is proportional to the spatial separation of the two transceivers. For an accurate one-way ranging measurement, the times of both transceivers must be closely synchronized because the value t.sub.s is determined by the transmitting transceiver while the value t.sub.r is determined by the receiving transceiver. In two-way ranging, each transceiver transmits a pulse, the first at time t.sub.1s and the second at t.sub.2s. The first receives the second's pulse at time t.sub.1r, and the second receives the first's pulse at time t.sub.2r. Even if the timers of both transceivers are not synchronized, the spatial separation is proportional to [(t.sub.1r -t.sub.1s)+(t.sub.2r -t.sub.2s)]/2, because the offset between the timers is removed by the subtraction. Consequently, the precise synchronization required for one-way ranging is not needed in two-way ranging systems.
Although a two-way ranging system avoids the synchronization problem in one-way ranging, each transceiver in a two-way ranging scheme must do more processing, that is, each transceiver must receive a pulse for each range it is involved in measuring. The times of arrival of the received pulses and time of transmission of the transmitted pulse or their differences must be telemetered to the controller aboard the boat for each shot point. For a transceiver involved in the measurement of many ranges, a lot of data must be processed. Consequently, only a transceiver with a high throughput can be used effectively in a two-way ranging system.
Therefore, one object of this invention is to provide a hydroacoustic transceiver capable of the high output rates required for two-way acoustic ranging without the need for accurate time synchronization.
If all the transceivers on a ranging system transmit on only one frequency, the only way to measure the various ranges is by time-division multiplexing, i.e., staggering the transmissions in such a way that no two pulses transmitted by different transceivers can arrive at any receiver simultaneously. Such a requirement, in addition to causing a transmit scheduling nightmare, results in a long time to measure many ranges, which causes errors in the position solution.
Another object of the invention is to provide a transceiver capable of transmitting and receiving hydroacoustic pulses having selected characteristics.
A further problem with acoustic ranging is errors caused by multipath interference. The straight-line path from transmitting transceiver to receiving transceiver is the direct path, which is the path defining the actual spatial separation. Other paths are due to reflections of the transmitted pulse off the ocean surface or floor. Depending on the differences in the lengths of the reflected paths with respect to the direct path, the reflected pulses may interfere with the direct pulse. Such interference can be destructive, preventing or distorting the detection of the pulse, resulting in an error in determining the time of arrival of the direct pulse. In addition, the shorter the transmitted pulse the less susceptible it is to multipath interference and the greater is its spatial resolution. It is well known in the art that the narrower the pulse, the wider the transmitter and receiver bandwidths must be. In other words, there is a tradeoff between resolution (pulsewidth) and bandwidth.
Wider bandwidths for each pulse of a given carrier frequency require that each channel in a frequency-division-multiplexed system be separated further. Accommodating a wide range of carrier frequencies is difficult in typical hydroacoustic transducers.
One way of squeezing more channels in a given transducer's bandwidth is by synthesizing narrow transmit pulses and detecting them using a matched-filter receiver. With a matched-filter receiver, it is possible to achieve a lower pulsewidth-bandwidth product than with ordinary receivers. A true matched-filter receiver, however, cannot be realized in the linear analog transceivers typically used. Consequently, analog transceivers must sacrifice resolution to enjoy the flexibility afforded by more channels or must sacrifice frequency flexibility to improve resolution.
One technique used with analog transceivers to avoid the multipath problem is to sequentially transmit pulses on different channels and analyze the transit times measured on each channel. The idea is that, for the same reflected paths, the interference between the direct and reflected pulses is different at different frequencies and that, at one of the frequencies, the interference will not be destructive and the range measurement can be made. This use of frequency diversity to solve the multipath problem takes more time, because more than one pulse must be transmitted by each transceiver to get a valid range measurement.
Therefore, it is a further object of this invention to provide a hydroacoustic transceiver operating on a number of efficiently packed channels and transmitting hydroacoustic pulses sufficiently narrow to minimize multipath interference.